Inorganic composite membrane comprising molecular sieve crystals

ABSTRACT

Inorganic composite membrane containing molecular sieve crystals, comprising a macroporous support to which molecular sieve crystals and modifications thereof have been applied substantially as a monolayer, said crystals and modifications thereof having been oriented so that, toa substantial extent, the pores of the sieve crystals form a significant included angle with the support surface, there being present between the crystals a gastight matrix, at least gastight to a degree sufficient under practical conditions.

This application is a continuation of our copending application, Ser.No. 08/098,287, filed Aug. 4, 1993, now U.S. Pat. No. 5,429,743.

This invention relates to an inorganic composite membrane containingmolecular sieve crystals and to methods for producing such a membrane.For separation on a molecular level, such as gas separation, vaporpermeation and pervaporation, mainly membranes on the basis of organicpolymers have been proposed so far for use on an industrial scale. Awide variety of macromolecular (almost exclusively organic) materialshave been found to be suitable for use as a membrane material.Reasonable separation factors can be achieved, and the throughput ofsuch membranes is sufficiently large.

However, these polymer membranes have the disadvantage of a relativelyshort service life. Owing to the sensitivity of the materials tosolvents (swelling) and the low stability at high temperatures, therange of application is limited. Moreover, regeneration by oxidativeremoval of impurities is not possible.

Also known are so-called ceramic membranes composed substantially ofinorganic materials, which, compared with polymer membranes, have theadvantage that they are resistant to high temperatures, so thatregeneration is possible, and moreover are relatively inert. Suchmembranes are usually produced starting from multi-layered systems, inwhich a relatively thick macroporous layer serves as a support for amicroporous top layer which is much thinner relative to the supportinglayer and exhibits the separation properties. The production of suchmembranes, in which the so-called sol-gel or dip-coating techniques canbe used successfully for providing the separating layer, is describedinter alia in the following publications: A. Larbot, A. Julbe, C.Guizard, L. Cot, J.Membr.Sci., 93, (1989), 289-303; A. Larbot, J.P.Fabre, C. Guizard, L. Cot, J.Am.Ceram.Soc., 72, (1989), 257-261; W.A.Zeltner, M.A. Anderson, "Chemical Control over Ceramic MembraneProcessing: Promises, Problems and Prospects", in: Proc. 1stInt.Conf.Inorg.Membr., (eds. J. Charpin, L. Cot), Montpellier, France,Jul. 3-6, 1989, 213-223; A. Leenaars, Preparation, Structure andSeparation Characteristics of Ceramic Alumina Membranes, PhD thesis,University of Twente, Netherlands, (1984); H.M. van Veen, R.A. Terpstra,J.P.B.M. Tol, H.J. Veringa, "Three-Layer Ceramic Alumina Membrane forHigh Temperature Gas Separation Applications", in: Proc. 1stInt.Conf.Inorg.Membr., (eds. J. Charpin, L. Cot), Montpellier, France,Jul. 3-6, 1989, 329-335.

A disadvantage of such ceramic membranes is that the separationefficiency is low. In most ceramic membranes developed so far,separation takes place on the basis of Knudsen diffusion. In that case,the rate of transport is inversely proportional to the square root ofthe molecular weight. The selectivity of the separation process issufficient only if molecules having widely divergent molecular weightsare to be separated from each other.

Improved insights have led to separation processes on the basis ofceramic membranes exhibiting material transport mechanisms other thanKnudsen diffusion, such as surface diffusion or capillary condensation:R.J.R. Uhlhorn, "Ceramic Membranes for Gas Separation; Synthesis andTransport Properties", PhD thesis, University of Twente, Netherlands,(1990). In the case of surface diffusion, use is made of differences inchemical and/or physical properties of the molecules to be separated.The surface of the separating (or active) part of the membrane ismodified in such a manner that one type of molecule is transported muchmore rapidly than the other as a result of a difference in surfacediffusion. However, the insight into the mechanism of surface diffusionis still poor, so that it is difficult to make appropriate use ofdifferences in chemical and/or physical properties.

In capillary condensation and multilayer diffusion, use is made of theformation of a liquid phase in the separating part of the membrane.Here, too, it may be advantageous to modify the surface of the membrane.Although the separation efficiency can be high, the implementation ofthe separation process is strongly bound by specific values of processparameters such as temperature and pressure, as a result of the vaportension of the condensing material.

Another drawback of the known ceramic membranes is that the pore sizedistribution is hard to control. Because the pores of the active layerare not uniform in size and shape, it is not possible to have such amembrane function as a molecular sieve. It has moreover been found to bevery difficult to prepare a microporous layer that is stable underprocess conditions.

The use of crystalline microporous materials renders it possible inprinciple to exactly adjust the pore size distribution on a molecularlevel. There is a wide variety of such materials, of which particularlythe zeolites (microporous aluminosilicates) are frequently used on anindustrial scale. Zeolites are now being used as adsorbent, ionexchanger and catalyst. Due to the molecular sieve properties, processeswith a high selectivity can be carried out. However, the molecular sieveproperties are optimally used only if these materials are arranged in amembrane configuration.

In the development and use of such membranes, it is of essentialimportance that information be available on mass transport by thezeolite crystals (cf R.M. Barrer, J.Chem.Soc. Faraday Trans., 86 (7),(1990), 1123-1130. Hayhurst and Paravar studied the diffusion ofalkanes, using a zeolite membrane configuration (A.R. Paravar, D.T.Hayhurst. "Direct Measurement of Diffusivity for Butane Across a SingleLarge Silicalite Crystal", 6th Int.Zeol.Conf., (Eds. D. Olson, A.Bisio), Reno, USA, Jul. 10-15, (1983), 217-224; D.T. Hayhurst, A.R.Paravar, Zeolites 8, (1988), 27-29). In this study, use was made of atwin silicalite crystal in an organic matrix and a low feed gaspressure.

Werner and Osterhuber studied the permeation through a faujasite type(NaX) single crystal, using a substantially higher feed gas pressure.(D.L. Wernick, E.J. Osterhuber, "Diffusional Transition in ZeoliteNaX: 1. Single Crystal Gas Permeation Studies", 6th Int.Zeol.Conf.,(Eds. D. Olson, A. Bisio), Reno, USA, Jul. 10-15, (1983), 122-130; D.L.Wernick, E.J. Osterhuber, J.Membr.Sci. 22, (1985), 137-146).

The most favorable configuration for a membrane having 10 molecularsieve properties is realized if the molecular sieve crystals form theonly separation between two fluids. In that case, molecules can passdirectly from the first (retentate) phase to the second (permeate) phaseonly via the micropores of the molecular sieve crystals.

It is difficult, however, to arrange molecular sieve crystals in amembrane configuration. Proposals are known where the molecular sievecrystals are included in a polymer phase (cf Dutch patent application8800889; European patent application 0 254 758, and U.S. Pat. No.4,740,219).

Further known are various ceramic membranes produced using molecularsieve crystals. Different methods have been proposed for includingmolecular sieve crystals in a macroporous support which either haveinitially been hydrothermally synthesized or are crystallized in situ inor on the support. Further, membranes have been prepared in which on amacroporous ceramic support an ultrathin layer of molecular sievecrystals in a ceramic matrix is dispersed. This has also been donewithout using a macroporous support (cf. European patent applications 0180 200, 0 135 069, and 30 0 265 018; U.S. Pat. Nos. 4,699,892 and4,800,187; Canadian patent 1,235,684 and Japanese patents 63291809 and60129119).

The above-mentioned ways of preparing membranes start from very smallsizes of the molecular sieve crystals.

Material transport through micropores proceeds very slowly and isinversely proportional to the thickness of the membrane. In general,therefore, active layers of a few micrometers or less are used. In manycases, the molecular sieve crystals are selected more than one ordersmaller than the thickness of the active layer.

A great disadvantage of using very small crystals is that it isvirtually impossible to realize the optimum configuration of themolecular sieve crystals in the membrane. This is caused by the poormanageability of such small particles. The passage through theseparating top layer requires that the pores of the crystals be inproper alignment in the direction of the material transport through themembrane. The possibility exists that this is the case only to a limitedextent, so that large parts of the membrane surface are not permeable.Moreover,,in practice, material transport along the molecular sievecrystals cannot be precluded completely, which causes a strong reductionof the selectivity.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph of a porous clay layer suitable for embedding ofcrystals deposited upon a two layer support member;

FIG. 2 is a photograph of a silicalite crystal embedded in a depositedclay layer;

FIGS. 3-5 are photographs of an alumina support member having depositedthereon a clay suspension containing embedded zeolite crystals and ahomogeneous gastight glaze film;

FIG. 6 is a photograph of a silicalite crystal incorporated in a claylayer on a support member with a glaze applied thereto;

FIG. 7 is a photograph of four (4) juxtaposed silicalite crystalsembedded in a layer of clay and disposed upon an alumina support memberto which a thin glaze film has been applied;

FIG. 8 is a more detailed photograph of the structure shown in FIG. 7;

FIGS. 9-12 are photographs of small holes present in a continuous glazefilms prepared in accordance with the present invention;

FIG. 13 is a photograph of a wide crack in the top layer of a membraneprepared in accordance with the present invention, the crack beingattributed to forced clamping in a measuring cell;

FIGS. 14 and 15 are photographs of bonding effected between a glazelayer and an alumina layer used in the practice of the presentinvention;

FIG. 16 is a photograph of the membrane of FIGS. 14 and 15 after thedeposition thereon of a thin glaze film;

FIG. 17 is a photograph of a membrane in accordance with the presentinvention showing that the deposition of a glaze powder under zeolitecrystals is avoided by appropriate disposition of crystals and supportmember; and

FIG. 18 is a photograph of a crystal embedded in a glaze matrix whichwas polished until the crystal surface was reached.

Therefore, according to the invention, a membrane with molecular sieveproperties is proposed, in which the abovementioned disadvantages do notoccur. The membrane according to the invention comprises a macroporoussupport to which molecular sieve crystals have been appliedsubstantially as a monolayer, between which crystals is present a matrixgastight at least to a degree sufficient under practical conditions.

In the membranes according to the invention, the orientation of themolecular sieve crystals on the support is important. In nature, a widevariety of molecular sieves are known, while intensive research has ledto a much larger number of synthetic molecular sieves. Each type has aspecific pore structure, and often the chemical composition is alsofixed to a certain degree. The morphology may be different for eachmolecular sieve, although it can generally be influenced. Finally, theparticle size for each molecular sieve is also adjustable to a certainmaximum size.

In the membranes according to the invention, the morphology, theparticle size and the pore structure of the molecular sieve crystals areimportant parameters. Molecular sieves may have a one-, two- orthree-dimensional pore structure. In the case of a three-dimensionalpore structure, in which the pores are equal in all three maindirections (e.g., zeolites A, X and Y), the particle size is important.Since the crystals having a regular morphology crystallize out (cubic:zeolite A, or octahedral: zeolites X and Y), orientation on the supportis of minor importance. To properly arrange such crystals in a membraneconfiguration, the crystal size is preferably at least 10 μm.

If, however, the pores of a type of molecular sieve extend only in twoor even one main direction (for instance, AlPO₄ -5, VPI-5, mordenite andNu-10), the crystals must be oriented on the support in such a mannerthat to a substantial extent the pores of the crystals form asignificant included angle with the support surface. If such a type ofmolecular sieve is arranged in the membrane configuration according tothe invention, the crystal morphology is of great importance. It hasbeen found that many molecular sieves preferably crystallize out in theform of needles, the pores being oriented exactly in the direction ofthe long axis. In that case, incorporation in a membrane according tothe invention becomes virtually impossible. In many cases, however, itis quite possible to influence crystallization in such a manner that themolecular sieve crystallizes out in a flat form, with the pores orientedexactly in the direction of the minimal size. It is preferred to usemolecular sieves of such a morphology, because then a relatively largesurface is covered with each crystal. Moreover, the thickness of theactive layer is thus reduced as much as possible.

For such types of molecular sieves, the thickness of the crystals ispreferably not less than 2 μm. The width and length of the crystals ispreferably at least 10 μm, so that the orientation on the support in thedesired direction can be properly realized. Although crystals in theform of sheets or tiles are preferred, this is not a prerequisite.

In the membrane according to the invention, therefore, in principle anytype of molecular sieve can be used. It is to be expected that for mosttypes of molecular sieves, crystals having a suitable morphology can beobtained. For different types of molecular sieves, the preparation oflarge crystals has already been extensively studied and described in theliterature. In this connection, reference may be made to, for instance,the following publications: J.F. Charnell, J. Crystal Growth 8, (1971),291. This publication describes the preparation of large crystals of themolecular sieve types A and X, the crystals of the A-type being cubicand of the X-type octahedral.

The preparation of a molecular sieve of the AlPO₄ -5 type (AFI; a porousaluminum phosphate) is described in U.S. Pat. No. 4,310,440 and in thepublication by S. T. Wilson et al. in J.Am.Chem.Soc. 104, (1982), 1146.The preparation of this type of molecular sieve is also described by S.Qiu et al in Zeolites 9, (1989), 440-444, according to which also verylarge single crystals can be formed. It is a disadvantage of thissynthesis, however, that the pores are in the longitudinal direction ofthe crystals. The preparation of an AlPO-type molecular sieve havingvery wide pores (VPI-5) is described by M. E. Davis et al. in Zeolites8, (1988), 362.

A special example is zeolite ZSM-5 (MFI) . This type of molecular sievehas been extensively studied, and different morphologies of this typeare known. Although this zeolite exhibits a three-dimensional porestructure, a pore direction can actually be indicated in two maindirections only: the socalled straight and sinusoidal channels. Thereare different publications indicating that the transport through thestraight and sinusoidal channels is not completely identical (forinstance, E. R. Geus et al. in "Zeolites for the Nineties", RecentResearch Report, Book of Abstracts of 8th Int.Conf. on Zeolites,Amsterdam, (1989), No. 135, 293-295).

The two most frequent morphologies of ZSM-5--the prismatic and cube ortile morphology--can be prepared in different ways. When crystals havingthe prismatic morphology are arranged in a membrane configurationaccording to the invention, there is a random distribution of straightand sinusoidal channels. When cubical crystals are used, a membrane withmainly straight channels can be produced, because there is one clearpreferred orientation. Therefore, with these two different morphologiesof ZSM-5 type crystals, membranes of dissimilar properties can beprepared.

U.S. Pat. No. 3,702,886 discloses the preparation of a molecular sieveof the ZSM-5 type. The preparation of an aluminium-deficient variant ofZSM-5 (silicalite) has been described by E.M. Flanigen et al. in Nature271, (1978), 512. The preparation of large cubical single crystals ofZSM-5 is dscribed by H. Lermer et al. in Zeolites 5, (1985), 131. Forthe so-called Sand synthesis for the preparation of prismatic crystalsof ZSM-5, reference may be made to the publication by M. Ghamani et al.in Zeolites 3, (1983), 155-162. The fluoride synthesis for thepreparation of prismatic crystals of ZSM-5 is described by J.L. Guth etal. in: "New Developments in Zeolite Science and Technology", Y.Murakami, A. Iyima, J.W. Ward (Eds), Proc. 7th Int.Conf. on Zeolites,Tokyo, Japan, Aug. 17-22, (1986), Kodansha, Tokyo and Elsevier,Amsterdam, 121-128. It has recently been found that by adding otherelements (for instance, boron) crystals of the ZSM-5 type can be formedwith the tile morphology (for instance, J. C. Jansen et al. "IsomorphousSubstitution of Si by B, Al, Ga, and Be during Crystallization of LargeSingle Crystals of Zeolite. Part I. on the Maximum Boron Content ofZSM-5", in "Innovation in Zeolite Materials Science", F. J. Grobet etal. (Eds), Elsevier, Amsterdam, (1988), 133-141; J.C. Jansen et al."Isomorphous Substitution of Si in Zeolite Single Crystals. Part II. onthe Boron Distribution and Coordination in B!-ZSM-5", in "Zeolites:Facts, Figures, Future", P. A. Jacobs and R.A. van Santen (Eds),Elsevier, Amsterdam, (1989), 679-688). In addition to the advantage ofone preferred orientation, the particle size distribution is very narrowand can be optimally adjusted. The yield of the synthesis is high, and apure product is formed. Finally, a pure silicon dioxide lattice can beformed by means of a socalled postsynthesis, which strongly reduces thesensitivity to clogging.

An example of a molecular sieve which, in the first instance, yieldedpredominantly crystals in the form of needles is mordenite. By means ofadaptation of the synthesis, however, it has been found possible tostrongly inhibit the crystal growth in the direction of the channels, sothat crystals having a suitable shape were obtained (cf P. Bodart et al."Study of Mordenite Crystallization III: Factors Governing MordeniteSynthesis", in "Structure and Reactivity of Modified Zeolites", P.A.Jacobs et al. (Eds), (1984), Elsevier, Amsterdam, 125-132).

In this connection, it is observed that German Offenlegungsschrift 38 27049 describes the preparation of zeolitic membranes, in which a fullycontinuous layer, which is not a monolayer, of zeolite crystals isformed on a porous support. According to the process described, thislayer is obtained by first making the surface of the support seedactiveand then dipping the support in a solution containing the components forforming the zeolitic material. The thus applied layer is then brought tocrystallization. This must be carried out several times. Apart from thefact that by using this method only a few types of zeolite layers can beapplied (only zeolite A is discussed in an example), it is difficult tocarry out the crystallization in a controlled manner such that a welldefined crystal film is obtained at the surface. For this reason, it ishardly possible to fully avoid material transport along the crystals,which adversely affects the separation selectivity. Moreover, formolecular sieves having an asymmetric pore structure, crystallizationmust be thoroughly controlled so that the crystals are correctlyoriented on the support.

In the membrane according to the invention, as stated before, acollection of molecular sieve crystals was spread as a monolayer overthe surface of a macroporous, in particular an inorganic, support.

The support must be sufficiently flat to orient the molecular sievecrystals in one plane. Different materials are suitable as acoarse-porous, inorganic support. Thus, a metal support starting fromsintered metal powder can be used, but so can oxidic (ceramic) supports.Different types of supports are commercially available. As a support, atwo-layered system is preferred. In that case, the coarse-porous part ofthe support gives the necessary support, and the flat orientation of themolecular sieve crystals can be properly realized on the relatively thintop layer.

A gastight ceramic matrix, at least sufficiently gastight for practicaluse, must be disposed between the molecular sieve crystals on thesupport, so that material transport is only possible via the microporesof the molecular sieve crystals. Requirements are further imposed on thechemical and mechanical properties of this matrix. Thus, the materialmust be inert under process conditions. Further, the matrix materialmust have a correct combination of properties (modulus of elasticity andthermal expansion coefficient), so that during the process conduct themembrane remains intact.

This invention also relates to a method of producing an inorganiccomposite membrane having molecular sieve properties. According to thisprocess, a layer, substantially a monolayer, of relatively largemolecular sieve crystals is applied to a macroporous support, betweenwhich crystals a gastight matrix is provided.

The object of the methods according to the invention is to obtain thehighest possible degree of coverage of molecular sieve crystals on thesupport. In this way, the maximum effective membrane surface isrealized. The form of the crystals plays an important part in theirapplication to the support. The sheet-like crystals of zeolite ZSM-5referred to above will be applied to the support with the correctorientation almost without an exception. In addition to the form of thecrystals, the uniformity of the particle size is also important. In thatcase, it appears that a very high degree of coverage can be realized.Moreover, in that case the diffusion path is equal in the wholemembrane, so that a membrane having very constant properties can beproduced. The particle size distribution can be properly controlled bymeans of the crystallization process. In addition, there is thepossibility of fractionation, for instance, by using sieves.Particularly in the case of single crystals, the use of sieves will beadvantageous, as has been demonstrated in the fractionation ofsilicalite crystals. It appears that due to the prismatic form of thismolecular sieve only the single crystals can pass through the smallestsieve openings (<38 μm) . Material grown together will generally be toobroad.

According to one method, an amount of molecular sieve crystals which issufficient and not too large to form a monolayer is scattered on thesupport. When subsequently, for instance by means of low-frequencyvibrations, a monolayer is formed, it appears that a degree of coverageof about 80% can be realized. An even higher degree of coverage can berealized by means of a liquid flow over a porous support saturated withthe same liquid, which support pushes up the molecular sieve crystals toform an almost continuous layer on the support.

It is also possible to treat the molecular sieve crystals, in the firstinstance, with a surfactant, so that the crystals obtain a surfacehaving a hydrophobic character. Thus treated molecular sieve crystalsremain afloat on water and are found to assume substantially ajuxtaposed position. A macroporous support can be disposed under thethus formed monolayer of molecular sieve crystals, after which the waterlevel is lowered to the upper side of the support. The crystals are thusapplied to the support in a high coverage.

In the preparation of the membranes according to the invention, it isgenerally advantageous to attach the molecular sieve crystals to thesupport to a certain degree before applying the matrix material. Thisattachment may be effected in different ways. When starting from amonolayer of loose molecular sieve crystals on an inorganic support, anattachment that is sufficient in many cases can be realized throughabsorption of water or a colloidal suspension of an oxide by the supportand then drying (optionally at elevated temperatures) of the entiresystem. The attachment, however, can be improved by applying to themacroporous support an ultrathin coating of an oxide or a mixture ofoxides which already liquefies at relatively low temperatures. A layerof, for instance, borosilicate glass (BSG) can be deposited on thesurface of an inorganic macroporous support by means of CVD (chemicalvapor deposition) techniques. Here, clogging of the macroporous supportdoes not occur, because the deposited layer is too thin for that.Subsequently, a monolayer of molecular sieve crystals is applied to thethus modified support in an otherwise similar manner, after which thetemperature is increased to the liquefying temperature of the mixingoxide. Upon cooling, a glass phase is formed again, fixing the molecularsieve crystals to the support.

In an alternative method of bonding molecular sieve crystals to thesupport, use is made of a silicone paste. Such material is viscoelasticfor a limited period of time. By pressing the molecular sieve crystalsinto the layer of silicone paste within this period, a proper attachmentis realized. During baking out of the silicone paste, a porous silicafilm is formed in which the molecular sieve crystals are properlyattached. This method is also suitable if a solution of a siliconerubber or a highly viscous silicone oil is spread as a blanket overmolecular sieve crystals and support. The polymer solution is preventedfrom penetrating into the pores of the macroporous support by fillingthe support with, for instance, water. The solvent is evaporated, and,after baking out, a similar silica film results. In principle, in thismanner other polymers can also be applied as a film, which arecompletely burned during baking out. In that case, any polymer film(e.g., polychloropropene or polybutadiene) may be applied. Such anattachment method may be useful if the actual matrix material can beapplied at low temperatures. An additional advantage is that the poresof the support are temporarily clogged, so that no matrix material canbe deposited between the molecular sieve crystals and the support.

According to a preferred method, the molecular sieve crystals areattached to the support using a highly diluted clay suspension.Molecular sieve crystals to some extent attached to the support areincorporated in a clay suspension by pouring an extremely diluted claysuspension over the support. After baking out, a ceramic layer isobtained which is thinner than the molecular sieve crystals, so thatthese protrude. It is also possible to apply the clay layer by means ofa dipping technique. Then, too, the starting material may be molecularsieve crystals that are already attached to some extent. In analternative method, the starting material is a clay suspension in whichthe molecular sieve crystals are already dispersed. In this case, thesuspension is spread over the surface of the macroporous support. Thedegree of coverage of the membrane surface is then properly adjustableby setting a high concentration of molecular sieve crystals. If asufficiently homogeneous layer cannot be obtained in one step, ahomogeneous layer can still be obtained by means of a clay suspensionapplied according to the first-mentioned method. In another preferredmethod, the starting material is a commercial alumina support(coarse-porous) which is provided with a thin clay layer according toone of the above methods.

The thus modified support is baked out, and a two-layered support isobtained, on which the molecular sieve crystals can be excellentlyoriented, for instance, by means of directed low-frequency vibrations.Subsequently, the whole pore volume of the support is filled with water,followed by a mild heat treatment. The molecular sieve crystals are thussufficiently attached to the support to properly carry out thesubsequent steps in the membrane synthesis.

After the molecular sieve crystals have been applied to the support as amonolayer and have optionally been attached thereto, the gastight matrixis applied to the support between the molecular sieve crystals.

According to the invention, different known techniques are suitable forapplying the matrix. A distinction is made between methods by whichmatrix material is applied as a blanket over both the support and thecrystals and methods by which it is selectively deposited between thecrystals. Preferably, deposition methods are used by which matrixmaterial is selectively applied between the crystals, because it is notnecessary, then, to remove part of the matrix material on the crystals.

By using generally known sol-gel techniques, also discussed in the aboveliterature, a thin layer of matrix material can be reproduciblydeposited. A great advantage of the sol-gel technique is the goodhomogeneity of the deposited material. The composition of the gel isdetermined during the preparation of the sol, in which the differentcomponents can be simply mixed on a molecular scale. Thus, sols ofmixing oxides can be simply prepared by mixing the corresponding metalalkoxides with a solvent and water. Similarly, collodial suspensions canbe prepared very homogeneously. Binders may be added to give the sol thedesired physical properties. In addition, so-called DCCAs (DryingControlling Chemical Agents) may be added. Thus, the drying process isbetter controlled, so that no cracks are formed in the film.

The sols may be applied in different manners to the support providedwith a monolayer of molecular sieve crystals. The simplest method is topour out the sol over the support surface. Because the viscosity of thesol has been set high by means of additives, the sol does not penetrateinto the support.

According to another suitable method, use is made of the so-calledspin-on technique (cf T. Bein et al. in Stud.Surf.Sci.Catal. 49, (1989),887-896), in which a flat support is rotated very rapidly. By using thismethod, a very homogeneous layer of matrix material can be deposited ona support.

According to yet another method, use is made of the known dip-coatingtechnique referred to above (cf A. Leenaars, "Preparation, Structure andSeparation Characteristics of Ceramic Alumina Membranes", PhD thesis,University of Twente, Netherlands, (1984); H.M. van Veen, R.A. Terpstra,J.P.B.M. Tol, H.J. Veringa, "Three-Layer Ceramic Alumina Membrane forHigh Temperature Gas Separation Applications", in: Proc. 1stInt.Conf.Inorg.Membr., (Eds. J. Charpin, L. Cot), Montpellier, France,Jul. 3-6, 1989, 329-335). According to this technique, a suitable sol iscontacted with the dry substrate for a period of time to be controlledvery accurately. At the surface of the substrate, a phase separation ofthe sol takes place, comprising absorption of the liquid into the poroussupport and deposition of the sol particles as a layer on the support.In the last-mentioned technique, there will be almost exclusivedeposition beside the molecular sieve crystals, because the phaseseparation will only take place on the surface of the macroporoussupport.

After drying the gel, a xerogel is formed having a very high surface ifthe sol composition is appropriately chosen. As a result, the gel isvery sintering active, so that during a heat treatment an irreversibletransition of the gel occurs and a dense film results. By properlyadjusting the composition of the gel and the heat treatment to eachother, a gastight layer of a metal oxide can already be obtained atrelatively low temperatures (400-500° C.) . By an appropriate selectionof the composition of a mixing oxide, the sintering properties can beimproved, because such materials are usually sintering active already atlower temperatures.

By employing a method utilizing the above-mentioned CVD technique(Chemical Vapor Deposition), a matrix can be deposited from the gaseousphase at elevated temperatures. With this method, too, it is possible todeposit layers of one metal oxide or a mixture of oxides. For nearlyevery metal, precursor molecules for carrying out the CVD process areavailable. Silicon dioxide films can be deposited, for instance, bymeans of the oxidation of silane or the pyrolysis of alkoxy silicates(for instance, the decomposition of tetraethyl orthosilicate; B.Delperier et al., "Silica CVD from TEOS on Fe/Cr/Ni Alloy", Proc. 10thInt.Conf. on CVD, The Electrochem.Soc., Pennington, N.J., (1987),1139-1146). However, for the production of a membrane according to theinvention, the deposition of borosilicate glasses will be preferred,because in that case the deposition temperature can be considerablylower. The process by means of the oxidation of hydride compounds(silane and borane) has long since been known (e.g., W. Kern, R.C. Heim,J.Electrochem.Soc. 117, (1970), 562-567). The decomposition of alkoxidesas the above-mentioned tetraethyl orthosilicate and trimethyl borate isadvantageous, however, not in the last place because of the fact thatsuch compounds are much less explosion-sensitive. Such processes havelong since been known too (for instance, P. Eppenga, et al., Journal dePhysique, Colloque C5, (1989), 575-584).

Because of the simple process conduct, the CVD process is preferablycarried out under atmospheric pressure. In alternative methods, forinstance, a plasma is used, so that these methods can be carried out atmuch lower temperature and, in many cases, at reduced pressure, becausethe reactant supply is limiting due to the low vapor tension of thereactants. The deposited layer may not yet be gastight after deposition.By means of a thermal posttreatment (optionally a hot pressingtechnique), however, a gastight layer can be formed in a simple manner.

In a special embodiment, use is made of the possibility of supplying thereactants separately. In that case, the process can be carried out inthe membrane module itself. This is sometimes referred to as ChemicalVapor Infiltration (CVI). By properly adjusting the pressure on bothsides of the substrate, it is possible to realize a very localdeposition. Such a process has already been studied for a long time inconnection with the development of solid fuel cells (SOFC) . A thin filmof yttrium-stabilized zirconia is deposited on a macroporous support asa top layer, the chlorides of yttrium and zirconium being presented onone side of the substrate and a mixture of oxygen and water on the otherside. In the first instance, the deposition proceeds according toabove-discussed CVD process. After the pores of the substrate havebecome clogged owing to the deposited layer, further growth takes place,because oxygen ions can diffuse through the deposited layer. Thus,layers having a thickness of 20 to 50 μm can be formed (cf U.B. Pal,S.C. Singhal, "Electrochemical Vapor Deposition of Yttria-StabilizedZirconia Films", in Proc.1st Int.Symp. on Solid Oxide Fuel Cells, TheElectrochemical Society, Vol. 89-11, (1989), 41-56; J.P. Dekker, N.J.Kiwiet, J. Schoonman, "Electrochemical Vapor Deposition of SOFCComponents", in Proc.1st Int.Symp. on Solid Oxide Fuel Cells, TheElectrochemical Society, Vol. 89-11, (1989), 57-66; Y. S. Lin et al., inProc.1st Int.Symp. on Solid Oxide Fuel Cells, The ElectrochemicalSociety, Vol. 89-11, (1989), 67-70; and N. J. Kiwiet, J. Schoonman,"Electrochemical Vapor Deposition: Theory and Experiment", in Proc. 25thIntersociety Energy Conversion Engineering Conference, Vol. 3, Paul A.Nelson, William W. Schertz and Russel H. Till (Eds), (1990), AmericanInstitute of Chemical Engineers, New York, 240-245.

In each of the above-mentioned CVD processes, very advantageous use canbe made of an applied porous intermediate layer which partly fills thespace between the molecular sieve crystals. Thus, the deposition ofmatrix material under the crystals can be avoided. This applies to thenormal CVD processes but especially also to the above-descrihed CVIprocess. In the CVD process, deposition on the molecular sieve crystalsis hard to avoid. In the CVI process, this largely depends on theprocess conditions. Selective removal of matrix material on themolecular sieve crystals is quite possible, however, by means ofpolishing or etching techniques, which will hereinafter be explained.

In a preferred method of applying the matrix, glaze powders are usedthat melt at low temperatures. An advantage of using such glazes is thegreat freedom of composition of the matrix material, so that an optimumcombination of material properties can be obtained. The glaze mustliquefy sufficiently to result in a gastight and properly adhering layerduring the heat treatment. The viscosity of the glass during the heattreatment must be sufficiently high, so that the glaze is only appliedto the macroporous support. Because eventually the membrane will also beused at high temperatures, the temperature during preparation must besignificantly higher than the process temperature. It is also necessarythat the regeneration can be carried out at a considerably lowertemperature than the preparation temperature. Suitable glazes are oftencommercially available.

The application of the glaze can be realized in many ways. For instance,a suspension of glaze powder can be applied over a monolayer ofmolecular sieve crystals on a macroporous support. Use can also be madeof "spray" techniques, while glaze powder can also be applied to thesupport in dry form. If the molecular sieve crystals are attached to thesupport sufficiently firmly, the glaze can be selectively applied besidethe molecular sieve crystals. In that case, a fine powder is appliedover the entire support in dry form, whereafter powder located on themolecular sieve crystals is swept off. In such a method, it is desirablethat the molecular sieve crystals be much larger than the powderparticles of the glaze.

Preferably, the glaze is applied using a strongly diluted glazesuspension. This method can be carried out directly in a macroporoussupport in module form, for instance a tubular module. The molecularsieve crystals may already have been bonded to the support in one of themanners mentioned. However, it is also possible to apply the crystals tothe support in situ from a suspension. In that case, the support iscompletely enclosed by a fluid phase. By allowing fluid to flow throughthe support continuously, the molecular sieve crystals are attached tothe support surface. The glaze suspension is then added to the fluidflow and the glaze particles are retained as a filter cake on the freesurface of the support. The fluid flow also fixes the glaze powder. Bythe accumulation of particles on the support, the pressure drop acrossthe module increases over time. This pressure drop can serve as ameasure for the thickness of the layer deposited. As soon as the layerhas a sufficient thickness, the addition of the glaze suspension isdiscontinued. The powder particles present on the molecular sievecrystals are removed by the still ongoing fluid flow, while in the glazepowder layer further densification occurs. The fluid is then absorbed bythe support, followed by a heat treatment.

According to a particularly suitable embodiment of the above-describedmethod, use is made of the specific advantages offered by the dipprocess. The molecular sieve crystals are first attached to themacroporous support, preferably using the above-mentioned claysuspension. Then a glaze suspension of very fine powder is prepared. Thehomogeneous dispersion of powder particles is obtained throughultrasonic vibration of the suspension and subsequently allowing thelarger particles to sink. The support, flat in this case, provided withmolecular sieve crystals, is dipped into the glaze suspension for a fewseconds by the side thereof on which the layer of glaze is to bedeposited. This so-called dipping must be carried out carefully so as toprevent complete submersion of the support. Through phase separation,discussed above, glaze powder is selectively deposited beside themolecular sieve crystals on the support. A thin layer of extremely thinpowder is formed, in which larger pores are clearly observable. Duringthe temperature treatment, sufficient liquefaction occurs for a propercontinuous glaze to be formed. The dip process can optionally be carriedout several times in succession with intermediate drying of thesubstrate. Even after baking out, it is possible to carry out the dipprocess once more, which can be used with great advantage as an in siturepair technique.

As explained hereinabove, in a number of methods of applying the matrixmaterial, this material is deposited as a blanket over both the supportand the molecular sieve crystals. In that case, by means of etching orpolishing, for instance, the matrix material is selectively removed fromthe molecular sieve crystals. These techniques are known per se.Depending on the flatness of the macroporous support and the particlesize distribution of the molecular sieve crystals, either the polishingor the etching method is chosen. During the polishing procedure, thecrystals are preferably additionally supported, for instance using aresin. A resin layer is applied to the top layer of the membrane,whereafter both the resin and the matrix material on the crystals areground off gradually. The remaining resin is removed through oxidationor dissolution.

When an etching method is used for the removal of the matrix materialfrom the molecular sieve crystals, this method may be a wet (chemical)or dry (via a plasma) etching method, depending on the compostion of thematrix material. Thus, for instance silicon dioxide can be removed in avery well controlled manner using an aqueous solution of hydrogenfluoride or using a plasma of a fluorocarbon compound such as CF₄ (Ch.Steinbruchel et al., "Mechanism of Dry Etching of Silicon Dioxide",J.Electrochem. Soc. 132 (1), (1985), 180-186), C₂ F₆ (T.M. Mayer,"Chemical Conversion of C₂ F₆ and Uniformity of Etching SiO₂ in a RadialFlow Plasma Reactor", J.Electronic Soc. 9 (3), (1980), 513-523), or CHF₃(H. Toyoda et al., "Etching Characteristics of SiO₂ in CHF₃ Gas Plasma",J.Electronic Mat. 9 (3), (1980), 569-584). As will be clear, when thesupport is finished, the support can be removed using a suitable methoddepending on the support used, so that a membrane film is obtained.

The membranes according to the invention can be used for any applicationfor which at present thermostable membrane configurations are proposed,and in particular for separations at molecular level. Because molecularsieves and in particular zeolites such as ZSM-5 and zeolite Y are usedin catalysis, the invention also relates to a catalytically activemembrane having molecular sieve properties.

For some decades now, research has been done into the use ofcatalytically active membranes. For an extensive review, reference ismade to V. T. Zaspalis, Catalytically Active Ceramic Membranes;Synthesis, Properties and Reactor Applications, PhD thesis, Universityof Twente, Netherlands, (1990).

Such a membrane with catalytic properties can be obtained according tothe invention by providing catalytic centres in the pores of themembrane and/or on the surface thereof prior to, during or afterproduction, using a technique which is known per se.

Here, the thermal stability of a membrane thus obtained is essentialbecause a great many catalytic processes take place at elevatedtemperatures (higher than permissible for organic polymers). Inaddition, it is often necessary to reactivate the catalyst (molecularsieve) in an oxidizing environment at elevated temperatures.

The catalytically active membranes according to the invention maycontain the conventional catalytically active molecular sieves as wellas molecular sieves modified, for instance, by isomorphous substitution,ion exchange or satellite formation.

The various stages of the production of the membrane according to theinvention will now be further explained in and by the following examplesand with reference to the accompanying photographs.

EXAMPLE 1

a) Preparation of uniform single crystals of ZSM-5/-silicalite.

Silicalite crystals were prepared using the Sand method (Zeolites 3,(1985), 155-162). The synthesis mixture consisting of 27.2 tetrapropylammonium bromide (TPABr, CFZ), 207.2 g sodium hydroxide (NaOH, Baker),167.4 g colloidal silicon dioxide (Ludox AS40, E. I. du Pont de Nemours)and 125.8 g water was heated for 162 h at 180° C. in a teflon-coatedautoclave. In the last phase of the synthesis, the crystals were in agel phase, which, using a caustic soda solution (0.5 M) was removed atapprox. 70° C. The crystals were calcined at 450° C. (temperatureincrease 1° C./min) . Then the crystals were fractionated by means ofsieves. The fraction smaller than 38 μn consisted exclusively ofprismatic single crystals.

b) Applying a monolayer of silicalite crystals to an α-alumina support.

Silicalite crystals were applied to an α-Al₂ O₃ two-layer support (NKA,Petten) of a diameter of 25 mm and a thickness of 2.5 mm. The supportsused consisted of a coarse-porous support layer (pore diameter 2-8 μm)of pressed α-alumina granules, to which a thin layer of α-alumina hadbeen applied via a slibcasting process (pore diameter 0.15 μm) . Singlecrystals of silicalite (prismatic; length about 200 μm, thickness andwidth about 30 μm) were applied in dry form to the top layer of thesupport, whereafter through vibration at low frquency (1-4 Hz) virtuallyall crystals were positioned side by side on the support. In thismanner, silicalite cyrstals with two orientations were obtained on thesupport.

Both the straight and the sinusoidal channels serve as pores for themembrane, without a preference for either of the two channels. Thenwater was absorbed by the support, in such a manner that both thecrystals and the support were completely moistened. The support wasdried at about 50° C. The zeolite crystals were now weakly bonded to thealumina support.

c) Embedding the crystals in a porous clay layer.

A very strongly diluted clay suspension was prepared by mixing 3.75 gclay suspension (kaolin; Porceleine Fles, Delft), 0.08 g quartz flourand 30 g water. The suspension was well homogenized using an ultrasonicvibrating bath. Of this suspension, 0.5 ml was applied to the dryalumina support, provided with zeolite crystals. The suspension spreadover the entire surface before the water penetrated into the support.The clay layer remained on the top of the support and as such embeddedthe zeolite crystals. The support was baked out in the following manner:1° C./min: 20°-95° C.; for 30 min at 95° C.; 3° C./min: 95°-350° C./min;2° C./min; 350°-900° C./min; for 60 min at 900° C.; 3° C.

Photograph 1 shows a picture of the clay layer on the two-layer support.Photograph 2 shows a silicalite crystal embedded in the deposited claylayer.

d) Applying the glaze film.

A suspension of 1.35 g glaze (lead borosilicate, melting point 800-900°C; Ferro B.V., Rotterdam) and 8.1 ml water was prepared. The suspensionwas homogenized for 5 min in an ultrasonic vibrating bath. The supportwas held in the suspension for 5 seconds and then dried in the air. Inorder to obtain a homogeneous, gastight glaze film, the followingtemperature programme was carried out: 5° C./min: 20°-95° C.; for 30 minat 95° C.; 1° C./min: °95-550° C./min; for 300 min at 550° C.; 3°C./min: 550-20° C.

Photographs 3-5 show the structure of the four-layer system formed inthis manner. Photograph 6 shows a silicalite crystal which has beenincorporated in the clay layer, whereafter a glaze film has been appliedto the support.

EXAMPLE 2

Silicalite crystals were synthesized in the same manner as in Example 1.The crystals were applied to the alumina support in the same manner andweakly bonded.

A similar clay suspension was used to apply a clay layer between thecrystals on the support. In this case, however, the clay layer was alsoapplied by the dip process by dipping for 5 seconds. The clay layer wasbaked out in an otherwise similar manner.

The glaze film was applied and treated thermally in the same manner asin Example 1.

Photograph 7 shows a section of four juxtaposed silicalite crystals onthe alumina support, embedded in a clay layer to which a thin glaze filmhas been applied. Photograph 8 shows the structure of the membrane inmore detail.

EXAMPLE 3

In this example it is demonstrated that it is also possible, as a firststep in the production of the membrane, first to modify the aluminasupport using the clay suspension. In that case it is not necessary touse a two-layer support.

In the same manner as described in Example 1, an accurately measuredamount of clay suspension was applied over an α-Al₂ O₃ supportconsisting of one layer. The support modified in this manner was bakedout in the same manner as described in Example 1 C) at 900° C. Then amonolayer of silicalite crystals was applied to the support in the samemanner as described in Example 1. The bonding of the silicalite crystalswas improved by baking out the still humid support according to thefollowing temperature programme: 1° C./min: 20°-95° C.; for 30 min at95° C.; 1° C./min: 95-550° C./min; for 120 min at 550° C.; 2° C./min:550-20° C.

The dip process with a glaze suspension and the thermal posttreatmentwere carried out in the same manner as in Example 1.

The advantage of the use of glaze powders appeared to be that the dryingstep--unlike the sol-gel process--is not in the least critical. Thepowder particles do not form a continuous layer but during thesubsequent temperature treatment liquefaction occurred to a sufficientdegree for a covering layer to be formed. The dip process in the case ofa glaze suspension appears to be little time-dependent as regards theamount of deposited material, which is an advantage over the dip coatingprocess using colloidal soles.

Thus, a smooth, continuous glaze film was formed, which properlyconformed to the irregularities of the support. In some supports theirregularities appeared to be too large, so that a few small holes werevisible in the glaze coating (photographs 9, 10, 11 and 12). It appearedto be quite possible to further close these holes with the same dipcoating process. Because the glaze suspension of water poorly moistensthe glaze surface, it becomes possible to deposit virtually exclusivelyglaze powder on the holes still present. The redundant glaze powder canbe removed using a water flow.

Using the dip coating technique, it also appeared to be possible torepair a composite membrane. Photograph 13 shows a wide crack (about 15μm wide) in the top layer of the membrane, which was the result offorced clamping in a measuring cell. The crack, which extended centrallythroughout the preparation, was completely filled with glaze powder. Thepreparation was baked out in the same manner as in Example 1 andappeared to close the crack completely.

Normally, the thin coating of glaze obtained exhibited no cracks, noteven during repeated heating and cooling. Indentation tests demonstratedthe much better mechanical strength of the thin glaze film relative tothe thicker glaze film which was obtained by pouring an amount of glazesuspension over the support. The thin glaze film which has been formedusing the dip coating technique appears to be much more homogeneous thanthe thicker glaze film formed through pouring of a suspension. This ispartly due to the fact that in the dip process exclusively very smallglaze powder particles are deposited.

EXAMPLE 4

The above experiments could also be carried out without using anintermediate layer. In that case, crystals were only weakly bonded tothe α-alumina support in the manner described in Example 1. Then a layerof glaze was applied by the dip process, whereafter the layer of glazewas melted by heating in an analogous manner to that in Example 1. It ispossible that in this manner glaze also penetrates between the crystalsand the support. The photographs 14 and 15 show the eminent bondingbetween the glaze film and the α-alumina top layer. It appears a verythin glaze film can be applied uniformly over the entire support surface(photo 16). Photograph 17 demonstrates that deposition of glaze powderunder the zeolite crystals can be prevented, provided crystals andsupport are sufficiently continuous relative to each other.

EXAMPLE 5

Silicilate crystals were obtained in the same manner as described inExample 1 and applied to a stainless steel support (Krebsoge), which hadbeen provided with a thin layer of silicone paste (Bizon). The supportwas baked out at 400° C. (temperature increase 1° C./min) . Then thesupport was placed in a bath and, using a level, arranged entirelyhorizontally. As much 1,1,1-trichloroethane was added as was necessaryto precisely fill up the support. Then 50 μm of a tetraethylorthosilicate (TEOS) sol (TEOS : water : ethanol=1 : 2 : 4) was pouredout over the support. By removal of the solvent (ethanol) from the solthrough evaporation and dissolution in the trichloroethane phase,gelation took place. The assembly so obtained was dried overnight andthen baked out at 500° C. (temperature increase 1° C./min).

EXAMPLE 6

Silicalite crystals were obtained and applied to an α-alumina support inthe same manner as in Example 1. The support was introduced into a CVDreactor (horizontal hot-wall reactor). Trimethyl borate (TMB) andtetraethyl orthosilicate (TEOS) were introduced into the reactor viaevaporators. The reactant flows were 50 sccm (TMB) and 200 sccm (TEOS),respectively. The unit sccm stands for cm³ /min at 25° C. and 1 bar. Thedeposition was carried out at 700° C. and 0.6 torr.

Deposition was performed for 6 hours, which yielded a borosilicate glasslayer of a thickness of about 4.8 μm.

In this manner a homogeneous glass layer was obtained which had beendeposited adjacent to and on top of the crystals. Some deposition hadalso taken place under the crystals.

Using inter alia polishing techniques which are known per se, matrixmaterial could be selectively removed from the top of the crystals. Thisis demonstrated by photograph 18, where a crystal which had beenembedded in a glaze matrix was polished until the crystal surface hadbeen reached. In this case polishing was done using a very fine aluminapowder (β-Al₂ O₃, 0.3 μm diameter; Union Carbide). In the case ofborosilicate films, etching techniques also proved eminently useful.

EXAMPLE 7

In the production of a membrane, substantially the method according toExample 1 was used, but now no clay layer was used, in view of the goodcompatibility of α-alumina and borosilicate glass (pyrex).

Again, the process started from a monolayer of silicalite crystals on amacroporous support. In this case, in an analogous manner to that usedwith the clay suspension, a suspension of pyrex glass powder (P5; mesh250) which had first been properly homogenized by ultrasonic vibration,was poured over the support (1 g pyrex P5 powder, 10 g demineralizedwater). On the support a powder layer was selectively formed beside thesilicalite crystals. The borosilicate film (melting point about 800° C.)was baked out at about 825° C. (heating rate 1° C./min).

EXAMPLE 8

A composite membrane was prepared in a manner as described in Example 4,with incorporation of an amount of crystals of the zeolite A type.Zeolites of the type A were synthesized according to Charnell (J. F.Charnell, "Gel Growth of Large Crystals of Sodium A and Sodium XZeolites", J.Cryst. Growth 8, (1971), 291-294). A mixture of sodiumsilicate (25.0 g), triethanolamine (56.0 g), sodium aluminate (20.0 g)and 360.1 g water was heated for one week at 75° C. Both single crystalsand twined crystals of zeolite A proved to have been formed with amaximum size (cube-shaped) of about 15 μm. Without further processing,these crystals were applied to a two-layer support (alumina; NKA,Petten) by spreading a suspension of crystals over the water-saturatedsupport using a nylon thread.The support was dried at 50° C., whereaftera glaze suspension (see Example 1) was applied by the dip process. Thefollowing temperature programme was then carried out: 1° C./min: 20-95°C.; for 30 min at 95° C.; 1° C./min: 95-550° C; for 60 min at 550° C.;2° C./min: 550-20° C.

We claim:
 1. An inorganic composite membrane comprising:(a) amacroporous support member, (b) a monolayer of molecular sieve crystalsapplied upon the support membrane, said crystals having pores forming asignificant included angle with said support member; and (c) asubstantially gastight matrix selectively deposited upon said supportmember in the area between the molecular sieve crystals.
 2. Membrane inaccordance with claim 1 wherein the molecular sieve crystals have a onedimensional pore structure and are selected from the group consisting ofAlPO₄ -5, VPI-5, mordenite, Nu-10 crystals and mixtures thereof. 3.Membrane in accordance with claim 1 wherein the molecular sieve crystalshave a two dimensional pore structure and are selected from the groupconsisting of ZSM-5, silicalite and mixtures thereof.
 4. Membrane inaccordance with claim 1 wherein the molecular sieve crystals have athree dimensional pore structure selected from the group consisting ofcrystals of zeolite A, zeolite X, zeolite Y, and mixtures thereof. 5.Membrane in accordance with claim 1 wherein the crystals are of athickness of at least 2 mm and a length and width of at least 10 mm. 6.Membrane in accordance with claim 1 wherein the crystals are appliedupon the matrix substantially with the same orientation.
 7. Membrane inaccordance with claim 1 wherein the molecular sieve crystals are atleast 10 mm in dimension.
 8. Membrane in accordance with claim 1 whereina porous fixing layer is applied between the support member and thecrystals prior to deposition of the gastight matrix.
 9. Membrane inaccordance with claim 8 wherein the fixing layer is formed from a clayselected from the group consisting of kaolin and baked-out siliconepaste.
 10. Membrane in accordance with claim 1 wherein the gastightmatrix is formed from a glaze, a borosilicate glass, an oxide or aceramic material.
 11. Membrane in accordance with claim 1 whereincatalytic centers are provided in the pores of the membrane and/or onthe surface thereof.
 12. Method for the preparation of a membrane inaccordance with claim 1 which comprises the steps of(a) applying amonolayer of molecular sieve crystals upon the surface of a macroporousinorganic support member, said crystals being oriented such that thepores of the crystals form a significant included angle with the surfaceof the support, and (b) applying a gastight matrix between saidcrystals.
 13. Method in accordance with claim 12 wherein a fixing layeris applied upon the surface of the support member prior to theapplication of said monolayer upon the support member.
 14. Method inaccordance with claim 13 wherein the fixing layer is derived from kaolinor baked-out silicone paste.
 15. Method in accordance with claim 13wherein catalytic centers are provided in the pores and/or on thesurfaces of said crystals.
 16. Method in accordance with claim 12wherein the gastight matrix is selected from the group consisting of aglaze, a borosilicate glass, an oxide or a ceramic material.
 17. Methodin accordance with claim 12 wherein any matrix material applied upon themolecular sieve crystals is removed by polishing or etching techniques.18. Method in accordance with claim 12 wherein the porous support memberis removed to obtain a self-supporting membrane film.
 19. A method forproducing the inorganic composite membrane recited in claim 1comprising:(a) applying molecular sieve crystals to a macroporoussupport such that said molecular sieve crystals applied on said supportform a monolayer which comprises crystals which are oriented such thatthe pores of said crystals form a significant included angle with thesurface of said support; and, (b) applying a substantially gastightmatrix between the crystals.
 20. The method recited in claim 19 whereina fixing layer build up from clay or baked-out silicone paste is appliedto the support surface.
 21. The method recited in claim 20 wherein thegastight matrix comprises a glaze, borosilicate glass, an oxide, or aceramic material.
 22. The method recited in claim 20 wherein matrixmaterial which has been formed on the particles is removed by polishingor etching.
 23. The method recited in claim 19 wherein said support isremoved to obtain a membrane film.
 24. The method recited in claim 17wherein catalytic centres are present or have been provided in the poresand/or the surface of said crystals.
 25. An inorganic composite membranecomprising a macroporous support, molecular sieve crystals, and a matrixbetween said crystals, wherein said crystals are present as a monolayerand are oriented on said support so that the pores of said crystals havea significant included angle with the surface of said macroporoussupport, said matrix between said crystals being substantially gastightand wherein said crystals have a one-dimensional or two-dimensional porestructure.
 26. The membrane recited in claim 25, wherein said crystalsare AIPO₄ -5, VPI-5, mordenite, Nu-10, or mixtures thereof when saidcrystals have a onedimensional pore structure and ZSM-5, silicalite, ormixtures thereof when said crystals have a two-dimensional porestructure.
 27. The membrane recited in claim 26 wherein said crystalshave the same orientation.
 28. The membrane recited in claim 26 whereinsaid molecular sieve crystals have a thickness of at least 2 μm and alength and width of at least 10 μm.
 29. The membrane recited in claim 26wherein said molecular sieve crystals have dimensions of at least 10 μm.30. The membrane recited in claim 26 wherein said membrane furthercomprises a porous fixing layer which is present between the support andthe crystals.
 31. The membrane recited in claim 30 wherein said fixinglayer is formed from clay or baked-out silicone paste.
 32. The membranerecited in claim 26 wherein said gastight matrix is formed from a glaze,borosilicate glass, an oxide, or a ceramic material.
 33. The membranerecited in claim 26 wherein catalytic centres are present or areprovided in the pores of said crystals.